Friday, September 28, 2012

Supernova Hunting

Somewhere in the observable universe, a star is exploding right now. Actually, something like 30 stars are exploding right this second, adding up to 2.5 million supernovae each day. That may sound like a ridiculously high number of exploding stars (If the universe is popping off supernovae so fast, then how do we have any stars left!?). Lets see if we can unpack it a bit. 

An average galaxy like our own produces roughly one supernova per century (I'll explain where this number comes from below). There are roughly 100 billion galaxies near enough to be observed by the Hubble Space Telescope (HST). If each of those observable galaxies gives us one supernova each century, then we expect about 100 billion supernovae every hundred years. One century is equal to about 3.15 billion seconds (that's about π x 109 seconds per year, as a handy way to remember it). So we divide those 100 billion supernovae over 3 billion seconds, and get roughly 30 supernovae per second.

August, 2010 (pre-Supernova) 
With so many supernovae blinking on every night, it is actually not too hard to find one of these objects. The three-step process is simple:
1. take a picture of the sky 
2. wait a few days or weeks, and take another picture
3. look for any new "stars" that weren't in the first picture
October, 2010 (see anything new?)

Subtracting off the August image
reveals the newly arrived SN Primo.
Stars and galaxies don't appear or disappear on the timescale of weeks (or years or centuries...) so there are very few astronomical objects that can appear so suddenly in between two pairs of images like that. Fast moving objects (like asteroids and comets) might move into your frame, but these are easy to sort out: take a third picture and you'll see that they keep moving. Anything that blinks on, stays in place, and then shows a steady rise and fall in brightness is most probably a supernova. The figure below shows two infrared images from HST.  The first was taken in early August, 2010, and the second was taken two months later, in October, 2010. The third image shows what happens when we subtract off the September picture: all the galaxies and stars are unchanged, so they get subtracted cleanly away, and we're left with just one new star. This particular supernova was the first one discovered in the CANDELS survey. Nicknamed "SN Primo,"  it is currently the most distant supernova of its kind. SN Primo and other stellar explosions we find with CANDELS will eventually be used to measure distances in the universe, helping us to understand the nature of the mysterious dark energy that is driving the accelerated expansion of space.

Supernova hunting is not limited to the professional astronomers with access to multi-million dollar observatories. Unlike many areas of physics, dedicated amateurs can and do make significant contributions to astronomy - especially in this sub-field of supernova science. The renowned Australian amateur Robert Evans has discovered over 40 supernovae himself, primarily using his own visual memory of the sky.  Lets take a moment to consider that, because this is really quite extraordinary: Rev. Evans was able to discover dozens of supernovae without using any of the careful image subtraction that astronomers rely on. He simply scanned the sky each night with his telescope, and looked for the single new pinpoint of light around a familiar galaxy that signals the death of another star and the start of a new supernova. We professional astronomers didn't get to be as efficient as Evans until the advent of robotic telescopes in the mid 90's.

Young amateurs are in on the supernova hunt, too. The unique object SN 2008ha was discovered by 14-year old Caroline Moore in upstate New York. In recent years this object has become a prototype for a whole new class of supernovae, which are still puzzling astronomers today. Alas, Caroline's record as the youngest person to find a supernova didn't last too long:  two years later SN 2010lt was discovered by Kathryn Gray, a 10 year old girl from Fredericton, New Brunswick in Canada.  
SN 2008ha was discovered by a 14-year-old amateur, and
astronomers now believe it to be the prototype of a new 

class of supernovae.  This picture was taken with the 2.2m
Telescope of the Calar Alto Observatory in southern Spain.
Image credit: Stefan Taubenberger, MPA
So there are 30 new supernovae every second, and we've got world-class telescopes and dedicated backyard astronomers on the hunt... but unfortunately we still don't actually see most of those supernovae. Some fraction are screened by dust, or hidden behind millions of other stars in the bright cores of their host galaxies. But most of the easily observable supernovae are missed simply because we aren't looking for them. To catch them all, we'd need a few million telescopes like HST observing every corner of the sky every day around the clock. We'll never have that, but there are some exciting new telescopes on the ground that can observe the sky much more efficiently than HST - although they don't go as deep or as distant. Amid the alphabet soup of astronomical acronyms, there's Pan-STARRS, PTF, and LSST, just to name a few. Eventually these wide-field surveys will really clean up in the local universe, detecting basically all the nearby supernova explosions.

This brings us back to the question of how do we know just how many supernovae are exploding each second. One critical piece of information is the rate of supernova explosions in an average galaxy. I stated at the top that this rate is about one supernova per century in a galaxy like our own Milky Way. We could measure that number by observing our own galaxy over a century and counting up the number of supernova explosions. That is painfully slow, and rather imprecise, but we can do effectively the same thing by watching a hundred galaxies for one year. But why stop there? It's far better to watch thousands or tens of thousands of galaxies over several years. Then we count up a large number of supernova detections, divide by the number of galaxies and the number of years and come up with the observed rate of one supernova per galaxy per century.   

This is precisely what we are doing with the CANDELS supernova survey - but with an important twist. The other wide-field surveys I mentioned above (like Pan-STARRS and PTF) are observing many thousands of galaxies each night, but they are limited to (relatively) nearby galaxies that are bright enough to observe in short exposures from the ground. The unique difference in the CANDELS survey is that we use very deep infrared imaging from HST. This allows us to look to higher redshifts (farther back in time) and catch supernova explosions within very distant galaxies in the early universe. Right now, our HST survey is the only program able to measure the supernova rate at a time when the universe was only about 3 billion years old. We can compare that observed rate from the early universe with the observed rate in the present-day universe to learn something about how the supernova population has changed. Do these early universe supernovae look the same as local supernovae? Are they exploding at the same rate as they do locally? These are the first questions that we're beginning to address with the CANDELS supernova program, and we hope the answers will help us understand more about these extraordinary events. 

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